Bugfix: Fixes crash on multiple update.

Signal/src/view controllers/MessagesViewController.m

@@ -687,6 +687,7 @@ typedef enum : NSUInteger {
         return;
     }
     
+    [self.collectionView performBatchUpdates:^{
         for (YapDatabaseViewRowChange *rowChange in messageRowChanges)
         {
             switch (rowChange.type)
@@ -719,8 +720,9 @@ typedef enum : NSUInteger {
                 }
             }
         }
-    
+    } completion:^(BOOL finished) {
         [self finishReceivingMessage];
+    }];
 }
 
 #pragma mark - UICollectionView DataSource